The present invention relates to industrial controllers and more specifically to controllers for injection molding machines.
In injection molding using an injection molding machine, a plasticized material is held in a "barrel" and forced under pressure, typically by means of a ram fitting within the barrel, through a nozzle in one end of the barrel. The plasticized material enters into a mold cavity under pressure where it solidifies into a molded part in conformance with the dimensions of the mold cavity. The part is then ejected from the mold and the process is repeated.
The injection molding process may be broken into a number of stages including: plastication, injection, packing and holding. Additional stages and subdivisions of these stages may also be employed. In the plastication stage, solid pellets of the molded material are typically fed into the barrel where they are melted and forced to the front of the barrel by rotation of a screw forming part of the ram. As the molding material is melted by mechanical action of the screw, the barrel begins to fill moving the screw and ram back from the nozzle. Control of the ram back pressure may be used to ensure the melted molding material is at the proper temperature and free from voids or air pockets.
In the injection stage, the rotation of the screw ceases and the ram is moved toward the nozzle to force the molding material through the nozzle into the mold cavity. The characteristics of the molding material, or of the mold, may require that certain parts of the mold cavity be filled at different rates. This may be accomplished by varying the speed or pressure of the ram during the injection stage.
In the packing stage, additional molding material is forced into the mold cavity to accommodate shrinkage of the molding material as it cools in the mold cavity.
In the holding stage, pressure is maintained on the molding material to control its density and/or flexibility. Control of the ram pressure during the holding stage may also prevent distortions of or depressions in the part as it cools. At the conclusion of the holding stage the molded part shrinks away from the mold cavity prior to ejection of the part.
The ability to accurately vary the ram pressure and velocity during the various stages of the injection molding cycle may be accomplished by a controller programmed with "profiles" comprised of sequential "segments" having different setpoints for the injection process. A given profile may have multiple segments and setpoints to allow the programming of complex molding pressure or velocity functions during a given stage of the injection process. At any given stage of the injection molding cycle, the user will provide a profile for controlling one process variable, (e.g., ram velocity, ram pressure, or cavity pressure) depending on the desired control strategy. The remaining process variables are either left uncontrolled or set to a baseline value.
The ability to use multiple segments in a profile requires that the injection molding machine respond rapidly to the setpoint in each segment. High speed control may be obtained by operating the injection molding machine in an open loop configuration. In an open loop configuration, the setpoint is converted into an electrical signal that when applied to the injection machine, for example, a hydraulic valve of the injection molding machine, produces a desired control variable value that will realize the setpoint's ram velocity or pressure. The conversion of the setpoint to an electrical value is based on empirical, a priori measurements of the valve's performance.
Although open loop control provides rapid response of the injection molding machine and ensures stability, the accuracy of the control is low. The reason for this is that the transfer function relating ram velocity or pressure to valve actuation is complex and subject to variations caused by numerous factors including the molding temperature, the mold type, and the composition of the plastic molding material.
For this reason, it is known to combine open loop control with "learning" in which the setpoint for each segment is compared with a fed back process variable value indicating the actual state of the ram at the conclusion of that segment. The difference is used to augment the control variable derived from the setpoint value for the next injection control cycle. Thus, if the ram fails to reach a desired pressure or velocity, the control variable is changed on the next cycle so that the ram's state at the conclusion of the segment in the next cycle better matches the original setpoint.
Another method for improving the accuracy of the open loop control is a hybrid open loop/closed loop system in which open loop values are provided to the ram and after a time, the loop is closed to eliminate residual errors. Closing the loop means that a measured process variable (e.g., ram position or ram pressure) is compared to the setpoint during a given segment and the injection molding machine receives a control signal proportional to the difference between the setpoint and the actual ram state.
Both of these methods of learning and combined open loop and closed loop control are described generally in U.S. Pat. No. 5,062,785 entitled: "Injection Molding Controller With Process Variable Learning" assigned to the same assignee as the present invention and hereby incorporated by reference.
While these approaches improve the control accuracy of the injection molding process, they are not wholly satisfactory in providing rapid and accurate ram control.